Every material ever employed in any corrosion application represents some sort of compromise. Not even the precious metals such as gold and platinum can combine all of the best properties possible. The austenitic 18Cr-8Ni alloys evolved as a balance between economic factors and requirements for resistance to various forms of corrosion, and for toughness, formability and weldability. As remarkable as the many variants of the 18Cr-8Ni type alloys have been they are especially vulnerable to failure in seawater or other chloride solutions by local corrosion or stress corrosion cracking.
There has been a somewhat concurrent and parallel evolution of nickel-base alloys, some of which do completely resist seawater and various chloride solutions. Due to the relative scarcity of nickel these alloys have, however, always remained quite expensive compared to stainless steels. They are simply economically not practical for many large tonnage applications.
Similarly recent developments have brought the newer ferritic, iron-chromium-molybdenum alloys to their present state of development and employment. The best of these alloys effectively resist failure in seawater and many other chloride environments. While their resistance to oxidizing substances is outstanding, they have much more limited utility for non-oxidizing conditions. As a result of further development their resistance to reducing conditions was somewhat broadened in variations containing small additions of nickel. But it was quickly learned that they had to have extremely low carbon and nitrogen contents. This meant that these alloys could not be produced by ordinary air-melting methods and would remain unavailable as cast articles.
Even before the ferritic iron-chromium-molybdenum steels were developed there existed a few early duplex alloys which combined austenitic and ferritic matrix structures. These alloys tolerated nitrogen and at least small amounts of carbon, were air-meltable and available as castings. More recently their utility and importance expanded rapidly with the understanding of the importance of small additions of nitrogen. Such additions reduce the unequal partitioning of chromium and molybdenum between the two phases and enhance resistance to pitting, crevice corrosion and stress corrosion failure, resulting in a class of stainless steels that combine some of the best features while sidestepping many of the undesirable characteristics of all of the prior alloy types.
These duplex alloys have found widespread use in the oil and gas industry, notably for line pipe, oil-gas separators, tubing and liners. They have been extensively used on the North Slope of Alaska for gas-gathering-line pipe to handle gas which contains large amounts of water and carbon dioxide which combine to form carbonic acid creating acidity conditions approaching a pH of 3.8. They are also used in a large variety of process equipment such as heat exchangers, tube sheets, tanks, pressure vessels, columns, fluegas scrubbers, shafts, pumps, valves, fittings and piping.
Present duplex alloys in widespread commercial use display critical crevice corrosion temperatures in 6% to 10% ferric chloride solutions of about 40.degree. to 73.degree. F. and critical pitting temperatures of about 95.degree. to about 125.degree. F. This gives an indication of their suitability for use in hot chlorides. A relatively new duplex alloy known as COR25 or Atlas 958 has a critical crevice corrosion temperature of about 100.degree. F. and a critical pitting temperature of about 160.degree. F. with about 18% elongation in standard tensile tests. Alloy 2205, the most widespread currently used duplex alloy has a 25% tensile elongation, 90,000 psi tensile strength and 65,000 psi yield strength but lower corrosion resistance.
Alloy 20Cb3 was once thought to be very cost effective for applications requiring resistance to stress corrosion cracking. This seemed promising, for example, in heat exchanger tubing at a relative cost of about 4 times that of common austenitic stainless steels, while the prior chloride resistant nickel-base alloys have costs about 7 to 8 times the standard stainless steels.
Later the ferritic stainless steels were widely hailed for their low cost due to relatively low strategic element contents, but their process costs remained very high. In the same tubing their performance often equals that of the Ni-base alloys at a relative cost of about 3 times standard 18Cr-8Ni. This is, of course, less even than 20Cb3 but not as low as that of the duplex alloys with their somewhat higher element contents, an example of increased costs due to strategic element content more than offset by reduced production costs.
And so the rapid current developments in duplex stainless steels rightfully deserve the considerable attention and utilization they are presently receiving. In many instances they combine the best properties of the austenitic and the ferritic stainless steels. More and more they are tending to combine the toughness, ductility, weldability and ease of production of austenitic stainless steels as well as the high yield strengths and relatively lower strategic element content of the ferritic stainless steels with the best corrosion resistance properties of both.
As remarkable as these newer duplex alloys are, their goals have been only partially met and there remains a vast demand for improved versions. Even within the group there has remained heretofore a polarization. At one pole are those duplex alloys which have offered the best corrosion properties but at the sacrifice of having the poorest mechanical properties of the group. At the opposite pole is the group of alloys optimizing the best mechanical properties but at the expense of having less corrosion resistance.